Assumptions:
160lb human, breathing in/out 6l air/minute (reflects slower resp
rate when sleeping)
ambient temp -5 deg C (Canadian winter camping eh?)
70% rel humidity

At this humidity there'd be 2.4 mg h2o/l of air inspired.
Exhaled air would be heated to body temp / 100% humidity

Heat up for 49.3 mg h2o / l
"Humidity deficit" = 49.3 mg h2o/l MINUS 2.4 mg h2o/l = 46.9 mg h2o/L

At 6 L air /minute there would be 281.4 mg h2o / min expired.

This works out to 135 ml h2o over 8 hours.

obviously less than your result for daytime rate, but your guess was
pretty good. But might be more for mouth breathers (what's that you
say BenT?)

alistair

On 8-Nov-10, at 2:25 PM, Mike S wrote:

At 05:00 PM 11/8/2010, Alistair Bell wrote...
> Michael, you'll love this, I have the data on how much water vapour a
> sleeping 160lb guy breathes out when asleep in - 5 C van.

Please do tell. A quick google says humans lose 350-600 ml/day of
water due to respiration. It would be lower during inactivity
(sleep), so I'm guessing in the 75-100 ml range for 6 hours. But if
it's -5C in the van, maybe it's more because you're shivering without
a heater.